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USS San Juan Specifications
Ben Gross, son of San Juan shipmate Lynden B. Gross, provided the following information.
He is gathering specifications on the ship so he can build a realistic & detailed model for his Dad.
Thank You Ben!

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      The following information was provided by Don Cavanaugh of Gibbs & Cox, Inc., the designers of the Atlanta Class.

      In the late 1930's, Gibbs & Cox, Inc. was tasked to design a new type of vessel for the US Navy. This class was developed to satisfy the need for a light displacement, high speed vessel whose mission was primarily combating large scale attack by aircraft, but which also possessed the ability to perform certain types of cruiser duty. The design consisted of many novel features, including the provision of an inner bottom extending to the second deck and following the contour of the outer shell. The side armor was of watertight reverted construction forming part of the watertight envelope of the hull. Armor protection was moderate, due to the weight limitation dictated by speed requirements, and consisted of side armor in way of the machinery spaces, bulkheads enclosing magazines, conning tower and steering engine room, with lighter protection on decks and on the boundaries off other vital areas. The propelling machinery was of improved design based on experience gained in the operation of destroyer machinery. Manufactured by Westinghouse Electric Corporation, each set of turbines consisted of one cruising, one high pressure and one double flow low pressure. The cruising turbine connected to the forward end of the high-pressure turbine rotor shaft through a single reduction gear.

      Reduction gears were locked train, double reduction type manufactured by DeLaval. The four 
boilers were designed by Babcock & Wilcox. Shaft horsepower was 75,000, maximum speed was 33 knots with a standard displacement of 6,000 tons, overall length of 541'9", and beam of 52'10". Armament consisted of sixteen 5-inch guns in twin mounts, three quadruple 1.1" antiaircraft machine guns, and two quadruple mount torpedo tubes.

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USS ATLANTA Class - Design History

      These cruisers were designed in the late 1930's under the constraints of the London Naval Treaty of 1936 which tried to place an 8,000 ton limit on cruisers with the abandonment of 8" gun (heavy) cruisers. Initial design concepts called for a "mini all purpose " Brooklyn class cruiser with dual purposes 6" armament but was quickly changed as it became apparent that a successful design could not be achieved on that displacement and a dual purpose 6" mount would not be ready for some time. It actually was developed and appeared in the Worcester class "large" light cruiser completed after WW2.
      The success of the new 5"/38 weighed heavily in the design discussions of the Atlanta class. The dual purpose 6"/47 gun would fire only about half as fast as a 5"/38, although its shell weighed roughly twice as much as the 5". The 5"/38 mount could fire 15, and with some well trained crews 20 rounds per minute while the 6" mount could fire 8 to 10 rounds per minute. Additionally, the value of very long range antiaircraft fire was open to speculation as the increased range of the 6"/47 was at the time outside the range of effective fire control. Hence the benefits of the dual purpose 6"/47 mount over a 5"/38 was not completely evident, and more importantly, could not be produced in time.
      Their initial purpose, contrary to popular belief, was not only that of an anti-aircraft cruiser but that of a small, fast scout cruiser that could operate in conjunction with destroyers on the fringes of the battle line in addition to the defense of the battle line against destroyer and aircraft attack. While they were not designed to "slug it out" with heaver ships, they were well suited to close surface action in bad weather (poor visibility) and to night actions, where their fast firing 5"/38's and eight 21" torpedos could be used to advantage.
      While these ships were conceived of as partly flotilla leaders they were armed with depth charges and sonar. Additionally, they were originally planned with 2 triple torpedo tubes which were to come from the Pensacola and Astoria class heavy cruisers. A change to quadruple tubes was made when tubes from the Sim's class of destroyers were removed for stability reasons and became available. One of the design features that generated the most interest in these ships is their use of true high pressure steam power plants. While some writings state that these ships were only capable of 32.5 knots at service weight, my father, who served aboard the Atlanta and was assigned to her prior to commissioning and hence on all trial runs, said that the ship did attain a speed in the high 30 knot range on trials. Since officially, the trial board stated that the Atlanta was good for about 85,000 SHP and 34 knots and she had achieved 33.67 knots on 78,985 SHP at 7,404 tons my fathers claim of a much higher speed seems probable given that the ultimate output was considered to be 90,000 SHP. I remember him mentioning that on a trial run at high speed the ship was put into a turn and water was washing over both the bow and stern as the ship executed the turn. 
      They appeared to be very maneuverable ships based on written stories as well as my fathers accounts of her in battle, especially against aircraft.
      Note that in later production ships the speed did in fact fall as weight increased. For example the Oakland made only 31.4 knots on 81,813 SHP at a higher weight of 8,150 tons.
The machinery plant reintroduced the alternating engine room and fireroom arrangement. This proved critical to the survivability of the ship to torpedo damage by providing protection against the total destruction of the powerplant.
      While there was much discussion and debate concerning the design and use of an all 5 inch main battery, they were generally well thought of and were ultimately ordered on April 25, 1939 to be completed in 1942. The Atlanta and Juneau were ordered from Federal (Kearny, NJ) and completed in the Brooklyn Navy Yard. 
      They were well designed ships and could withstand a great deal of punishment. 

There were 8 ships in this class.
USS Atlanta (CL-51) sunk by explosives after suffering heavy damage,11/13/42 
USS Juneau (CL-52) sunk by Japanese torpedoes, 11/13/42 
USS San Diego (CL-53) decommissioned 11/04/46, scrapped 1960 
USS San Juan (CL-54) decommissi0oned 11/09/46, scrapped 1961 
USS Oakland (CL-95) decommissioned 7/01/49, scrapped 1959 
USS Reno (CL-96) decommissioned 11/04/46, scrapped 1962 
USS Flint (CL-97) decommissioned 5/06/47, scrapped 1966 
USS Tucson (CL-98) decommissioned 6/11/49, scrapped 1971 
      Three follow-on ships (Juneau Class) were commissioned in 1946 and were distinguished from the Atlanta Class by a reduction of the superstructure height by one level, a reduction in the distance between the stacks, and a substantial increase in the antiaircraft batteries. This class had an array of various types of radar antennae installed on the fore and main masts.
USS Juneau* (CL-119) decommissioned 7/23/56, scrapped 1962 
USS Spokane (CL-120) decommissioned 2/27/50, scrapped 1973 
USS Fresno (CL-121) decommissioned 5/17/49, scrapped 1966 
*The Navy often reused names of lost ships.

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5"/38 Guns

United States of America 
5"/38 (12.7 cm) Mark 12 

The 5"/38 (12.7 cm) Twin Mount Mark 32
1. Trunnion28. Elevating arc (fixed to gun slide)
2. Gun port shield29. Training gear motor
3. Training gear handwheel30. Sight-setter's seat
4. Right gun carriage31. Training connecting shaft
5. Training indicator regulator32. Sight-setter's indicator
6. Training gear box, B-end33. Fuze-setting indicator regulator
7. Training gear box, A-end34. Trainer's Telescope
8. Barbette (fixed to ship)35. Trainer's seat
9. Ventilating system motor36. Rammer pump
10. Ready-service projectile stowage in handling room37. Case ejector chute
11. Projectile hoist (mount)38. Voice tube
12. Projectile hoist (ship)39. Ammunition hoist motors, under captain's platform
13. Powder hoist (mount)40. Mount captain's platform
14. Powder hoist (ship)41. Hatch
15. Ready-service powder stowage in handling room42. Elevating gear motor
16. Base ring (training rack on inside)43. Checker's telescope
17. Air intake compartment44. Pointer's seat
18. Case ejection chute door on back of shield45. Pointer's telescope
19. Air vent trunk46. Pointer's foot firing pedal
20. Auxiliary case-ejection port, in back of shield47. Elevating cross-shaft
21. Case ejector48. Fuze-setter's seat
22. Roof hatch counterbalance49. Roof hatch
23. Mount captain's blast hood50. Rear access door
24. Open sight51. Foot rungs
25. Air intakes52. Side access door retaining hatch
26. Rammer motor53. Side access door
27. Right gun slide

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How the 5"/38 crews operated By Dick B. - Updated 11 September 2001 

      The 5"/38 gun mounted in Destroyers was carried in single and twin mounts. The Trainer sat on the right side of the gun, with a vision port/sight and the Pointer on the left, also with a vision port/sight. The Trainer moved the mount in bearing (azimuth) while the Pointer moved it in elevation. Both stations had 'follow the dial' instruments for Director control or by Main Battery Plot, and the means for local control, which meant the Pointer and trainer, actually aimed the guns. This was true of both single and twin mounts. The seats were those perforated steel tractor type which got pretty uncomfortable after a while. The Director, of course, was located above the Bridge and had the rangefinder and radar gear. Four men, including an officer, manned it. Main Battery Plot was located below, right aft of the Mess decks, in the IC Room, with the main gyro. Guns could be fired from the Director, Plot or locally. It was the practice in most ships for all firing keys to be closed at the command "Fire" so that the circuit would be completed, no matter what. 
      On twin mounts, projectile hoists were located between the guns, serviced by the upper handling room, immediately below the mount. Projectiles were carried nose down, the fuse being set automatically by the mechanism in the hoist. Projectile men were trained to wait until the last possible instant to remove the round from the hoist, as the fuse setting was constantly being adjusted. 
The projectile weighed around 55 pounds and was painted and stenciled according to its purpose. They had a brass rotating band toward the base, which was sharp and occasionally cut one's palm as the round was handled. 
      It was the drill to grasp the projectile firmly at the base with the right hand (for the right hand gun) pulling it into the left hand about midway along the ogive. It was raised to a 'Port Arms' position across the chest, then laid in the tray ahead of the powder case. The Projectileman had, of course, to wait for the Powder man. Once all was set, the Projectile man hit the rammer lever with his right hand, in passing en route to the hoist to grab another round. The hydraulic rammer stuffed the packet up the spout and tripped the breech stop, allowing the vertical breech to slide up and close. The breech slid in bronze rails and there was a fair amount of polished brass and gleaming steel, necessary for the mechanical functions. Paint could foul or otherwise gum up the works, so nothing important was painted. 
      The Powder man stood immediately to the rear of the Projectileman, and received his goods through a scuttle in the deck plate, shoved up from the handling room below. The cartridge case came with a protective steel fitting over the primer, called a 'butterfly', which the Powderman had to knock off as he pulled the case up from the scuttle. This required a certain dexterity of the wrist, as the butterfly had a small ring handle on it, to help the loader lift the 35-pound charge. One didn't want to flip the butterfly off before having solid control of the cartridge, you understand. Having done his gymnastics, it remained to throw the case onto the tray, steadying it with the left hand, while the first loader manipulated his projectile. This also required some athletic ability, as the gun was wildly training and elevating, trying to track a 'fast mover. Immediately to the Powder man's right rear, the Hot Shell man stood, armed with a huge Asbestos mitt, to catch the fired casing and discharge it through the port provided in the rear of the gun house. These, being propelled from the breech at really healthy rate, could be a serious embarrassment to a gun crew, should this worthy fail, and allow them to ricochet around the interior of the mount. He had to be very fast and very accurate. He only got the one chance, you see. Normally the gun fired and ejected instantly. 
      The Gun Captain stood to the right rear of the gun, on a little elevated stand, supervising this orchestrated chaos. His was the responsibility for manually operating the breech in the event of power or hydraulic failure and commanding the crew in local control. 
      The Mount Captain commanded the mount, and operated the sound powered phones, passing orders and giving firing data as needed, and controlling the mount in local control. He perched on an elevated throne at the rear of the gun house between the guns in a twin mount, with a hatch he could stand in, the better to see his targets, etc. 
      In a twin mount, firing against aircraft, in a lively ship coursing through any sort of sea at all, life in the gun house was intense at times. It was invariably hot, an acrid odor of powder fumes permeated the place, a thin veil of smoke filled the air, and, for some reason, the unfired powder cases and projectiles had this peculiar sour smell, which burnt the throat and eyes. Imagine trying to maintain footing on heaving deck plates slick with hydraulic oil mist; hustle 55-pound high explosive shells into a gun now vertical, now at forty five degrees and now vertical again, all in a third of a second, for the accepted standard was twenty rounds a minute, bar nothing. 
      As the gun elevated, the breech end descended smartly into a well in the deck plates. It was not unknown for gunners to lose legs and feet as the heavy breech dropped down at an unfortunate moment. It has been known that gunners were mashed into chili by the breech, as a result of an untimely lurch. There was no warning or time for one, really, one had to keep his wits about him. 
The hoist hung under the gun, into the upper handling room. That space did not move with the mount, as does a turret, but merely with the ship. The compartment is circular, and holds all the ready service ammunition in racks along the bulkhead. The Shell man handles the projectiles and the Powder man the cases. In his case, he simply grabs one and shoves up through the scuttle to the Powder man above. All charges are the same. 
      The Shell man, however, has to grab the selected round from an assortment around the compartment and hustle it to the hoist, set it in the cup properly, lest he jam the hoist, and see it on its way. Both are working in a stifling hot, heaving space filled with the noxious odors and, I might add, no hope of surviving if something goes amiss. 
      Below them, in the Magazine, a crew keeps the handling room supplied with the necessaries, manually humping the projectiles and powders in conditions even worse than those above them. 
I know of one ship where Mount 52 fired just as Mount 51 opened the breech of the right hand gun, the mounts were so trained and pointed, that a flashback occurred from the muzzles of Mount 52 into Mt 51. Powder was ignited and all 17 men were killed instantly. Had a projectile gone off, the ship could have been lost. The Navy subsequently built some sort of stops into the firing circuits to prevent a recurrence. 
      The guns' crews are made up of Seaman branch sailors, mostly from the deck force. Bosun's Mates make up the gun crew's petty officers, in the main. Gunner's Mates are the 'tech's but also serve as Mount Captains and sometimes, Gun Captains. The Director is manned by Fire Controlmen, as is Plot. The Gunnery Officer may be in the Director or in Plot.

      Unquestionably the best Dual Purpose gun of WWII, this weapon was designed to arm the new destroyers being built in the 1930s. The 5"/38 (12.7 cm) was used on nearly every major US warship built between 1934 and 1945 and was still being used on new construction as late as the 1960s. It was also used on many auxiliaries and smaller warships as well as US Coast Guard vessels. This 
standardization, unique in any navy, greatly helped the logistical supply situation of the Pacific War. 
There were some teething troubles when this gun was introduced in 1934, but long before the start of WWII they were considered to be highly reliable, robust and accurate, a reputation they retained 
even after the end of the war when the 5"/54 (12.7 cm) series of weapons were introduced. 
      These guns were hand-loaded, but power-rammed which gave them a high rate of fire and made them capable of loading at any angle of elevation, both necessary qualities in an AA weapon. The earliest mountings as used on USS Farragut (DD-348) were pedestal mounts with shell and cartridge hoists located on the deck behind the gun mounting. However, starting with USS Craven (DD-380), a new base-ring mounting with integral shell and cartridge hoists on the axis of the mounting was introduced. This type of mounting meant that shells and cartridges could be passed directly to the gun's breech at any angle of training, thus improving the rate of fire. Most subsequent designs including all twin mountings were similar, although a simpler base ring type lacking hoists was introduced in 1943 for use on auxiliary vessels. The introduction of VT AA shells in 1943 made this weapon an even more potent AAA gun. 
      These guns were introduced to the British Royal Navy in 1941-1942 when HMS Delhi was rebuilt in New York Navy Yard. The British were impressed with the combination of the 5" (12.7 cm) gun and Mark 37 Fire Control System and tried to purchase additional units, but the rapid ramping up of US warship building prevented any diversion. The Mark A prototype for this gun was created from a cut-down 5"/51 (12.7 cm) Mark 9, the only version of that famous weapon that used semi-fixed ammunition. 
      Of autofretted monobloc construction and used a semi-automatic vertical sliding wedge breech mechanism. The gun barrel was secured to the housing by interrupted threads, thus allowing easy barrel replacement. About 8,000 of these weapons were produced between 1934 and 1945.

      Early units of the Atlanta Class light cruisers carried sixteen 5"/38 (12.7 cm) guns which gave them the heaviest AAA broadside of any USN warship.

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San Juan Specs - Page Two

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